1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and more particularly to a method of forming an alignment mark used in lithography, which provides the alignment mark possible to be detected even when a metal film or the like, which is opaque to a visible light, is present between a photoresist and a lower layer pattern.
2. Description of the Prior Art
The semiconductor device formed on the semiconductor substrate is, in general, made by repeated steps of forming an interconnection in a multi-layered layer, forming a contact plug to connect an upper interconnection with a lower interconnection, and the like. For instance, in order to connect a lower layer interconnection with an upper layer interconnection which is formed above the lower layer interconnection through an insulating film, the following steps may be taken.
Firstly, after a layer for the lower layer interconnection is formed, a lower layer interconnection pattern made of a photoresist is formed by means of lithography on the layer for the lower layer interconnection. Next, using this photoresist as a mask, the layer for the lower layer interconnection is worked upon and then the remaining photoresist is removed to form the lower layer interconnection. Next, an insulating film is formed over the entire surface, and again by means of lithography, a contact hole pattern made of a photoresist is formed on the insulating film. The contact hole pattern is formed so that the contact hole pattern is located and aligned over the lower layer interconnection. Next, using this photoresist as a mask, the insulating film is worked upon and then the remaining photoresist is removed to form the contact hole. The contact hole is subsequently filled with a conductive substance, and thereby a contact plug is formed. Next, a layer for the upper layer interconnection is formed over the entire surface, and again by means of lithography, an upper layer interconnection pattern made of a photoresist is formed thereon. The upper layer interconnection pattern is formed so that the upper layer interconnection pattern is located and aligned over the contact plug. Next, using this photoresist as a mask, the layer for the upper layer interconnection is worked upon and then the photoresist is removed to form the upper layer interconnection. In the result, the upper layer interconnection is connected to the lower layer interconnection through the contact plug.
In a series of the steps described above, the lower layer interconnection and the contact plug as well as the contact plug and the upper layer interconnection must be aligned so that their respective contact faces may be off each other. To conduct these alignments, alignment marks for achieving required superimpositions are formed both on the reticles (the masks for exposure) on which a circuit pattern is formed and on the semiconductor substrate. While the alignment mark on the reticle is formed, in advance, at the time of reticle formation, the alignment mark on the semiconductor substrate is formed in a region other than an element formation region, as the manufacturing steps proceed.
In recent years, demands for further miniaturization and higher performance for the semiconductor device have led to frequent application of the planarization onto the surface by means of the CMP (Chemical Mechanical Polishing), the extensive use of non-transmissive metal materials for the interconnection and the employment of a thinner film for the mark layer to constitute the alignment mark. These factors have made the detection of the alignment mark formed on the semiconductor substrate difficult.
One example of such a case that the detection of the alignment mark is made difficult as mentioned above is described in details below, with reference to a cross-sectional view of a structure of a memory cell section of a DRAM (Dynamic Random Access Memory) shown in FIG. 1.
In a prescribed region of a semiconductor substrate 1, there are formed an element isolation regions 2 as well as a source 9 and a drain 10, each of the source 9 and the drain 10 is made of a dopant diffusion layer. On the semiconductor substrate 1, a gate insulating film 3 is formed, and thereon a word line is formed which consists of a first silicon film 4 made of polycrystalline silicon, a metal film 5 made of tungsten or the like, a silicon nitride film 6, a silicon oxide film 7 and sidewall insulating films 8. After the word line is formed, a second silicon film is formed over the entire surface, and contact plugs 12 and 13 are formed, which respectively connect with the source 9 and the drain 10. A first interlayer insulating film 11 is then formed over the entire surface, and the planarization by the CMP method is applied thereto so as to expose the top surfaces of the contact plugs 12 and 13. Next, a second interlayer insulating film 14 is formed and a bit line contact hole 25 is made through the second interlayer insulating film 14 so as to reach the contact plug 13. Next, a bit line contact plug is formed by filling up the bit line contact hole 25 with a titanium nitride film 15 and a tungsten film 16. Following that, a bit line 17 made of a tungsten film or the like is formed so as to connect with the bit line contact plug. Then, a third interlayer insulating film 18 is formed, and a capacitor contact plug 19 is formed to run through the interlayer insulating films 14 and 18 and to be connected with the contact plug 12. Subsequently, a fourth interlayer insulating film 20 is formed and a deep hole to expose the top surface of the capacitor contact plug 19 is formed. By these formations, a capacitor consisting of a lower electrode 21, a dielectric 22 and an upper electrode 23 is formed, and thereby a memory cell of a DRAM is formed.
In the aforementioned DRAM memory cell, if the spacing of the word lines is sufficiently provided, the bit line 17 can be directly connected with the semiconductor substrate and, therefore, the alignment mark used in lithography at the time of formation of the bit line 17 may be made of the word line, as described in Japanese Patent Application Laid-open No. 2001-36036. On the other hand, when the spacing of the word lines becomes too narrow, it is difficult that the bit line 17 is directly connected with the semiconductor substrate and the connection of the bit line must be made indirectly through a contacting face that is raised to the level of an upper layer through the use of a contact plug, as shown in FIG. 1. The requirement for high standard alignment in such a case necessitates an alignment mark to be formed on the same layer as the contact plug at the time of the bit line formation.
A method of forming a bit line, in which the aforementioned alignment mark formed on the same layer as the contact plug is used, is described below.
FIG. 2 shows one example of a planar pattern for an alignment mark which is to be formed on the same layer as a contact plug. Inside the alignment mark section 102 with a width L of 180 μm, there are disposed nine rectangular alignment marks 101. The width A of the rectangular alignment marks is 6 μm, the interval B therebetween is also 6 μm and the length C thereof is 45 μm. In respect that the widths of both of the foregoing word line and bit line are 0.1 μm or so, it is evident that the alignment mark section occupies a substantially large area.
FIG. 3 shows a cross-sectional view of a structure of an alignment mark section at the stage where the bit line 17 is formed in the memory cell section in the DRAM of FIG. 1. When the formation of the bit line has been completed in the memory cell section of FIG. 1, a gate insulating film 3, a first silicon film 4, a metal film 5, a silicon nitride film 6 and a silicon oxide film 7 have been laminated on the semiconductor substrate 1 in the alignment mark section. Then, a silicon film is formed also in the alignment mark section at the same layer as the silicon film for forming the contact plug in the memory cell section. After that, while contact plugs 12 and 13 are formed by means of lithography and dry etching in the memory cell section, the silicon film in the alignment mark section is also worked upon, concurrently, with the alignment mark pattern shown in FIG. 2 being used, to form an alignment mark 13b. The alignment mark is herein presented as a single bar for convenience.
Next, an interlayer insulating film 11 is formed over the entire surface, and the planarization by the CMP method is applied to the surface thereof so as to expose the top surfaces of the contact plugs 12 and 13 in the memory cell section shown in FIG. 1 as well as that of alignment mark 13b in the alignment mark section shown in FIG. 3. Next, an interlayer insulating film 14 is formed and a bit line contact hole 25 is formed in the memory cell section. For the formation of the bit line contact hole 25, the alignment mark 13b is utilized. Next, by filling up the bit line contact hole 25 with a titanium nitride film 15 and a tungsten film 16, a bit line contact plug is formed. After that, a bit line layer made of a tungsten film or the like is formed, and then a bit line 17 is formed by means of lithography and dry etching.